The present disclosure relates to the use of the inherent high current of the inertial homopolar generator (HPG) discharge pulse to metal joining by the homopolar pulse welding (HPW) process. In this process, the faying surfaces of two workpieces in contact are preferentially heated as the current traverses the associated interfacial voltage gradient. High force is applied to forge the workpieces together when the proper temperature has been reached.
The advantages of the HPW process include a short welding time (from one to two seconds, independent of cross-sectional area), uniform heat generation throughout the section, a narrow heat-affected zone, and good strength retention. No filler metal is required, and welding can generally be accomplished in air with no preheating or shielding. Also, flash is minimal and relatively little axial upset is required, thereby minimizing material loss.
In accordance with this patent application, an experimental 10-megajoule (10-MJ) laboratory HPG to weld 6 in SHC 80 APIX-52 high strength steel pipe was successfully carried out.
Homopolar pulse welding (HPW) is an upset welding process that uses the unidirectional HPG current pulse to heat preferentially the interface between two workpieces which are lightly loaded mechanically but are in solid contact, and then increases the axial force to forge the workpieces together without melting them. Heat generation is initially concentrated at the interface of the confronting workpieces due to constriction resistance at the faying surfaces. Interface pressure is kept low early in the pulse to maximize the I.sup.2 R heating caused by the contact voltage drop. Ideally, the interface and the adjacent bulk material will have heated to forging temperature just as the pressure is increased. The workpieces continue to upset either to material refusal or until a mechanical stop in the welding fixture is reached. The amount of upset is typically between 0.20 and 0.25 inch (0.5 and 0.6 cm), regardless of the weld cross section.
Because of the proximity of the electrical contact leading edges (located as close as 0.5 in. (1.3 cm) on either side of the interface), the heat-affected zone is small compared to that obtained using many other processes. The comparatively high heating and cooling rates and the short time at weld temperature minimize grain growth and undesirable metallurgical reactions. Finally, because of the unidirectional nature of the pulse, current distribution and the resulting temperature distribution are relatively uniform over the cross-section, so that large, irregular sections can be successfully welded.
Photomicrographs of the weld line in a typical welded high carbon steel have shown that the weld line is inconspicuous, but can be detected by the local presence of some alpha veining (ferrite regions) in boundaries of pearlite grains. Presumably, the veining resulted from some superficial decarburization that occurred on the faying surface early in the weld heating cycle, within the second or so before coalescence took place. There was substantial grain refinement in these welds, as can be seen by comparison with the parent metal microstructure. The grain refinement should be beneficial to the properties of the weld. The weld microstructures, apart from the veining, consist of fine pearlite and possibly some bainite, which indicates that the cooling rates at 1,300.degree. F. (700.degree. C.) were between about 900.degree. F. and 1,800.degree. F./min (500.degree. and 1,000.degree. C./min). It was noted that these natural cooling rates are sufficiently low that martensite does not form in the weld zone. This is consistent with the observations of in-plant flash welding of similar materials, in which the welds were being allowed to cool naturally. Therefore, it is concluded that in homopolar pulse welding of carbon steel materials, martensite is not likely to create a problem, but if this conclusion should prove to be in error, there are means for controlling the cooling rates of welds by controlling generator excitation during the pulse.
Experience has shown that the welding electrical contacts need to be clamped onto the smooth, scale-free surface of the workpiece with a minimum pressure of about 2,000 lb/in..sup.2. Otherwise, the contact voltage drop at the electrical contacts/workpiece interface can result in sufficient local heat generation to damage the member (generally the workpiece) that has the lower thermal conductivity. The possibility of local overheating of the workpiece is greater for irregular sections, in which less peripheral length is available for electrical contact placement. For Area 90 A-A rail, the electrode contacts were rectangular chromium-copper blocks 2.0 in. (5 cm) long. This geometry resulted in a nominal electrical contact area current density of 25 kA/in..sup.2 (3.1 kA/cm.sup.2) and a leading edge linear current density of 58.2 kA/in. (22.9 kA/cm), higher by factors of 2.5 and 3.5, respectively, over those used in previous welding of pipe.
This disclosure teaches a homopolar pulse welding fixture which addresses and solves many of the problems identified in the prior art. This fixture is capable of welding tubular products having outside diameters in the 65/8- to 75/8-in. (16.8- to 19.3-cm) range, cross-sectional areas from 6 to 12 in..sup.2 (38 to 77 cm.sup.2), and has been used to produce successful demonstration welds of 65/8 in. OD.times.0.427 in. wall (16.8.times.1.1-cm) API X-52 line pipe with a cross-section of 8.5 in..sup.2 (54.8 cm.sup.2). The major problem areas that were addressed in the design of this fixture are:
(1) insufficient electrical contact-to-workpiece interface pressure; PA0 (2) workpiece burning under the leading edges of the electrical contacts; PA0 (3) insufficient mechanical gripping force; PA0 (4) prevention of buckling of the hot workpieces between the mechanical grips; and, PA0 (5) improving rigidity of alignment of the workpieces. The solutions to each of these problems as applied to pipe and rail welding are achieved by the present invention. PA0 (1) the laterally rigid electrical contacts; PA0 (2) the mechanical slips being located and actuated by an annular wedge ring; and, PA0 (3) enclosure of the entire welding fixture within a single rigid case or housing.
The contact pressure between the electrical contacts and the workpieces is very important in the HPW process. Large current pulses on the order of 70 kA/in..sup.2 of weld area (10 kA/cm.sup.2) are fed through the contact surface between the workpiece and the electrical contacts. Currents of this magnitude require high gripping pressures to prevent excessive workpiece heating under the contacts.
For a weld current of 70 kA/in..sup.2 .times.8.5 in..sup.2 or 595 kA, the average current density under the contact would have been 25 kA/in..sup.2 (3.9 kA/cm.sup.2) in typical prior art fixture designs. This high current density requires an efficient method of loading the contacts against the workpiece or otherwise local overheating of the workpiece will occur under the contacts.
The fixture of the present invention provides increased electrical contact area and clamping pressure between the electrical contacts and the workpieces. The total electrical contact area is 92.8 in..sup.2 (235 cm.sup.2) for the above total weld current, thereby providing a current density under the electrical jaws to 6.4 kA/in..sup.2 (1 kA/cm.sup.2). The electrical contact clamping actuators include six electrical contact shoes which provide an area of 15.5 in..sup.2 each (100 cm.sup.2) and is actuated by a single 1-8 UNC socket-head cap screw. Torquing of these bolts to their rated seating torque of 1,000 lb. ft (1,350 N.m) generates an axial force of 70,300 lb (313 kN), which corresponds to an electrical interface contact pressure of 4,500 lb/in..sup.2 (31 MPa). It has been found that this amount of contact pressure was not needed with the dual-material contact shoes of this invention, and that the contacts required only 300 lb.ft (400 N.m) of torque to function properly.
The present invention solves the problem of workpiece burning under the leading edge of the electrical contacts. In earlier welding fixtures, the contacts were copper blocks clamped onto the workpiece surfaces. When a weld is made, the discharge current from the HPG would flow from one set of electrical contacts into the workpiece, across the weld interface, into the second workpiece, and then out through the second set of electrical contacts. The current naturally tries to flow through the path of least resistance, and so it would remain in the lower-resistance electrical contact material for as long as possible. This caused the current to enter the workpiece primarily along the leading edge of the electrical contact shoes, i.e., the edge closest to the weld interface. This caused very high current densities at these leading edges. It was determined through previous experience with other welding fixtures that leading edge contact burning can be prevented if the leading edge linear current density, the total peak weld current divided by the linear dimension of the leading edge of the contact, is kept below 20 kA/in. (8 kA/cm). Comparing this value to the calculated leading edge linear current density in a prior art rail welding fixture of over 50 kA/in (20 kA/cm) predicts that there would be considerable leading edge burning. Assuming that the electrical contact on the 65/8-in. OD pipe to be welded in the fixture could be designed to utilize about 89 percent of the pipe circumference of 20.8 in. (53 cm), leading edge current density would be 32 kA/in. (13 kA/cm). This value, although lower than the one predicted, is considerably higher than the desired maximum value and led to the discovery that a potential contact leading edge burning problem was involved.
The solution to this problem was to design and build a set of electrical contacts according to this invention that would force the current to enter the workpiece uniformly under the contact area, or at least to enter the workpiece at more than one effective leading edge. The designs to accomplish these two solutions are set forth in the present disclosure. The preferred variable-resistance material selected to cause the current to enter the workpiece uniformly under the electrical contact is not currently commercially available, however, it appears feasible to manufacture a material of this sort by using variable-composition powder metal technology. It also appears possible to manufacture this material by centrifugally casting a metal alloy with different density constituents such as copper and tungsten. For welding the 6-in API X-52 line pipe with a resistivity of 19 .mu..OMEGA.-cm, a 5-in. (13-cm) long variable-resistance sleeve with a low-end resistivity of 2 .mu..OMEGA.-cm would need to increase to about 73 .mu..OMEGA.-cm. These resistivity levels correspond to the resistivity of ETP copper and 304 stainless steel, respectively. The number of different resistance materials required would be determined in accordance with this invention by considering the required number of effective leading edges needed to achieve the desired 20 kA/in. maximum allowable leading edge linear current density. For example, from previous calculations, in order to decrease the 32 kA/in. current density predicted for the X-52 pipe to less than the desired 20 kA/in., two effective leading edges would be necessary. Using equal areas of the two different-resistance materials and choosing ETP copper as the lower-resistance material, leads to a desired resistivity in the high-resistance material of 309 .mu..OMEGA.-cm. This resistivity is too high for commonly available materials, so the design was modified to have unequal areas of the materials. The area ratio of 4 to 1 was chosen for the low-resistance material to high-resistance material ratio. This led to a required resistivity of 123 .mu..OMEGA.-cm in the high resistance material, which corresponds to the resistivity of Hastelloy F. This differential area design increases the current density under the high-resistance material to 14 kA/in..sup.2, which proved to be satisfactory. Testing of this design showed no evidence of contact burning under the leading edge of the electrical contacts.
Homopolar welding of continuous length workpieces, such as pipe or rail, requires that the workpiece be rigidly gripped tightly enough to transfer the forging force necessary to complete the weld into the workpiece through a frictional grip. Previous welding fixtures attempted to do this with mechanically-set clamps which required the use of short deadheaded workpieces before they could produce satisfactory welds. Typical forging pressures of 20,000 to 30,000 lb/in..sup.2 (140 to 200 MPa) across the full weld cross-section produced welds of excellent quality. This forging pressure yields forging forces on 8.5 in..sup.2 (55 cm.sup.2) of weld area of between 170,000 and 255,000 lb (750 to 1,000 kN). These forces must be communicated from the welding fixture to the workpiece without damaging it.
In accordance with this invention, the solution of this problem was to incorporate the slip designs used in the oil industry for gripping pipes and casing. These slips typically use multiple rows of serrated-tooth dies set into self-actuating wedges. When used in the oil field, these slips are set by dropping them around the pipe and into a tapered bowl. This design causes the force that is being held to actuate the slips. The harder one pulls on the pipe, the tighter the slips grip. Typical slip ratings for 65/8 in. OD pipe are 375- and 500-ton (3.3- and 4.4-MN) load capacities. The pipe welding fixture and the rail welding fixture of the embodiments of the invention set forth in the drawings of this disclosure employ 48 gripping blocks set into six separate slip wedges to grip each workpiece. These six slip wedges are radially compressed against the pipe by a hydraulic wedging ring. The force from the hydraulic upsetting cylinder is transferred into the slip wedges through a bearing ring to prevent the forging force from creating excessive hydraulic pressures in the actuating cylinder. The slip wedge design worked very well and has been tested to forces in excess of 500,000 lb (2.2 MN) with surface marking on the pipe being less than 0.030 in. (0.75 mm) deep. This unexpected result has eliminated the problem of mechanical grip slippage that was seen in earlier weld fixture designs.
Another problem area which the welding fixture of this invention successfully addressed was that of buckling of the workpiece between the mechanical grips during the forging action. This problem has been noted on prior art welding fixtures and occurred because of the use of dual off-axis hydraulic cylinders to generate the forging force. The forces produced by these cylinders often were of unequal magnitude, especially during the dynamic portion of the forging action. Consequently, there was a possibility of imposing eccentric loads on the workpieces, causing the onset of buckling at lower loading values than those predicted for non-eccentric loads. Another difficulty discovered through use of prior art rail welding fixtures arose from the relatively large distance between the mechanical clamps and the lack of any rigid lateral restraint of the workpieces between these clamps. Assuming an effective diameter of the rail to have been about 51/2 in., then the 29-in. length between the mechanical clamps gave a column L/D ratio of 5.27 to 1.
The solution to this buckling problem was accomplished with the present novel pipe welding fixture by the provision of three design features. The first novel feature was to use a single annular upsetting cylinder with the workpiece located on the centerline of the cylinder. This eliminated any possibility of eccentric forging forces on the workpieces. The second novel feature was to close-couple the mechanical clamps. The distance between the clamps on the fixture of the present invention is only 17 in. (43 cm), which corresponds to a column L/D ratio of 2.6 to 1 for the 6-in. pipe. The mechanical grips themselves are designed to have a relatively large L/D ratio of 2.1 to 1, thereby providing a rigid cantilever support of the workpiece. The third novel feature as a means for preventing workpiece buckling between the mechanical grips by making the electrical clamps radially rigid, thus providing significant lateral restraint to the workpieces between the grips. These methods and apparatus achieved by the present invention solved the problem of workpiece buckling.
The problem of maintaining good mechanical alignment of the workpieces during the welding process is closely related to the problem of preventing eccentric workpiece loading and subsequent buckling. The novel solution set forth herein is the provision of a rigid lateral positioning and support structure for the workpieces that concurrently allows for axial motion required during the forging action. This was accomplished in accordance with this invention by the incorporation of several of the previously described design innovations, including:
Prior art heavy-section welding fixtures often rely on multiple precision-machined ways to provide lateral rigidity while providing for axial movement. Although this type of design is functional, the relative lateral stiffness of the machined ways is much less than that of the heavy-wall large-diameter enclosure used to provide mechanical alignment in the present welding fixture. The use of these alignment solutions has produced a fixture which has shown no noticeable misalignment through the various welding programs.